DF4

DF4

1. Bond enthalpies

   • Chemical bond = electrical attraction between atoms or ions
   • Bond enthalpy = Qunatity of energy required to break a particular bond in a molecule
   • Bond enthalpy will be a negative value if exothermic
   • Greater the bond enthalpy the stronger the bond
   • Equilibrium bond length = Attractive and opposing forces with in bond are equal
     • Shorter bond length = stronger bond
   • Double bond = high B.E Triple bond = V.high B.E
   • Average bond enthalpies used; exact value is dependent on particular compound bond is found.
   • Bond enthalpies measured indirectly using enthalpy cycles

2. Usefull links

   • http://www.lisgar.net/magwood/sch4u%20bonding%203.htm
   • http://www.chemguide.co.uk/physical/energetics/sums.html

3. Breaking and making bonds

   • CH4 + 2 O2 = CO2 + 2 H2O
     • ΔH = -890 KJmolmol-1
     • Is breaking bonds and creating new ones
       • Worked out by looking at structure of reactants and products
   • Always refers to breaking a bond in a gaseous compound
   • Combustion of methane = exothermic
     • energy in bond-breaking steps < energy in bond-making
   • Not all have to be broken to form new ones

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